1. Field of the Invention
The present invention relates generally to an ATM (Asynchronous Transfer Mode) network, and in particular, to an apparatus and method for multiplexing voice packets, efficiently using bandwidths of communication links connected to an IMA (Inverse MUX Assembly). More particularly, the present invention pertains to a technique of controlling an output rate of ATM cells delivered from an AAL2 (ATM Adaptation Layer 2) switch to an IMA in an ATM network using the IMA where voice, low-rate data, and high-rate data coexist.
2. Description of the Related Art
In a conventional ATM network, logical connections are established in correspondence with physical connections between a source and a destination. A data unit of a predetermined size (53 bytes) and format is delivered over the logical connections. The data unit is called an ATM cell. An ATM system can be connected to another ATM system using trunks like E1. In an ATM network supporting high-rate data service, trunks are multiplexed using an IMA to deliver a large amount of data.
FIG. 1 is a block diagram illustrating a conventional ATM communication network using IMAs. Referring to FIG. 1, a transmitter 10 and a receiver 20 are connected via TMAs. The transmitter 10 includes voice sources 101, an AAL2 switch 102 connected to the voice sources 101, low-rate data sources 103, an AAL5 framer 104 connected to the low-rate data sources 103, high-rate data sources 105, an AAL5 framer 106 connected to the high-rate data sources 105, and an ATM switch 107 with an IMA, that is, connected to the IMA, which is included into the switch. Thus, each switch has an IMA module, and the connection between switches is accomplished via the connection between IMA modules that are included in switches. The receiver 20 includes an ATM switch 108 with an IMA, an AAL2 switch 109 connected to the ATM switch 108, voice destinations 110 connected to the AAL2 switch 109, AAL5 deframers 111 and 113 connected to the ATM switch 108, and low-rate and high-rate data destinations 112 and 114 connected to the AAL5 deframers 111 and 113, respectively. AAL2 creates an ATM cell out of small-size voice packets, and AAL5 segments a long data stream into ATM cells.
In operation, the ALL2 switch 102 multiplexes short voice packets of a variable length into an ATM cell of a fixed size in order to suppress internal fragment caused by small traffic transmission in view of the use of the fixed cell. Internal fragment is a phenomenon that leads to performance degradation when small traffic is delivered in a fixed-size frame, not filling the entire frame, and thus utilizing part of a transmission link. The AAL5 framers 104 and 105 convert long data packets of a variable length into ATM cells of the fixed size.
The ATM switch 107 receives ATM cell streams from the AAL2 switch 102 and the ALL5 framers 104 and 105, and distributes multiplexed ATM cells of the ATM cell streams to a plurality of multiplexed physical links using the IMA. The physical links include a T1 transmission line (1.544 Mbps), an E1 transmission line (2.048 Mbps), a D3 transmission line (34 Mbps), and an STM (Synchronous Transfer Mode) transmission line. The following description is made in the context of the E1 transmission line, by way of example.
The receiving ATM switch 108 delivers the ATM cells received through the physical links to corresponding destinations. The AAL2 switch 109 recovers voice packets from an ATM cell received from the ATM switch 108 and delivers them to the voice destinations 110. Similarly, the AAL5 deframers 111 and 113 recover data packets from ATM cells received from the ATM switch 108 and deliver them to corresponding low-rate and high-rate data destinations 112 and 114.
FIG. 2 is a detailed block diagram of the AAL2 switch 102 and the ATM switch 107 illustrated in FIG. 1. Referring to FIG. 2, the AAL2 switch 102 includes an AAL2 multiplexer (MUX) unit 201 and an AAL2 demultiplexer (DEMUX) unit 202. The ATM switch 107 includes a MUX unit 203 and a DEMUX unit 204. The AAL2 MUX unit 201 has an input queue 301 for storing voice packets received from voice sources, and a MUX 302 for creating ATM cells out of the voice packets, as illustrated in FIG. 3.
In general, the AAL2 MUX unit 201 forms an ATM cell out of voice packets according to the rate of an output link or output port of a main board. If the output port has a high rate, the AAL2 MUX unit 201 processes the voice packets at high speed, and if the output port has a low rate, it processes the voice packets at low speed.
The reason for using AAL2 in the ATM network is to minimize internal fragment of a fixed-size ATM cell by forming the ATM cell out of a plurality of short packets through multiplexing. However, when a high-rate output port is used and an ATM cell is created at the rate of the output port, as soon as voice packets arrive at the input queue 301 of the AAL2 MUX unit 201, they are formed into ATM cells and then delivered to the high-rate output port. Each of the voice packets is eventually transmitted in one ATM cell.
In this case, AAL2 multiplexing offers no benefits and bandwidth waste results. Particularly, when the bandwidth of a communication line through an IMA is maximized, the bandwidth loss becomes severe. Multiplexing received voice packets after a predetermined time delay can solve this problem. However, this method is also based on a fixed output rate and thus an unnecessary time delay is produced when there are a small number of voice users.